Semiconductor device and manufacturing method of the same

ABSTRACT

A semiconductor device includes a buffer layer formed with a semiconductor adapted to produce piezoelectric polarization, and a channel layer stacked on the buffer layer, wherein a two-dimensional hole gas, generated in the channel layer by piezoelectric polarization of the buffer layer, is used as a carrier of the channel layer. On a complementary semiconductor device, the semiconductor device described above and an n-type field effect transistor are formed on the same compound semiconductor substrate. Also, a level shift circuit is manufactured by using the semiconductor device. Further, a semiconductor device manufacturing method includes forming a compound semiconductor base portion, forming a buffer layer on the base portion, forming a channel layer on the buffer layer, forming a gate on the channel layer, and forming a drain and source with the gate therebetween on the channel layer.

BACKGROUND

The present technology relates to using a two-dimensional hole gas,generated in a channel layer by piezoelectric polarization of a bufferlayer, as a carrier of the channel layer.

A compound semiconductor-based field effect transistor (FET) having aGaAs-based or other compound semiconductor layer offers high electronmobility, thus providing excellent n-channel frequency characteristics.Among FETs using an n-channel employed for high-frequency bands are HEMTand JPHEMT (refer, for example, to Japanese Patent Laid-Open No. Hei11-150264). HEMT is an abbreviation for High Electron MobilityTransistor, and JPHEMT an abbreviation for Junction PseudomorphicHigh-Electron-Mobility Transistor.

HEMT is an FET using, as a channel, a high-mobility two-dimensionalelectron gas induced at a semiconductor heterojunction interface. JPHEMTis an FET that provides an electron mobility higher than HEMP bytolerating a certain degree of lattice mismatch. JPHEMT is an FET thatoffers improved gate forward voltage (turn-on voltage) by using a pnjunction as a gate.

Some of such FETs using an n-channel employ, as a carrier, atwo-dimensional electron gas produced on the side of an electron travellayer at the heterojunction interface between an electron supply layerand the electron travel layer as a result of piezoelectric polarizationand spontaneous polarization between the electron supply layer andelectron travel layer (refer, for example, to Japanese Patent Laid-OpenNo. 2010-074077 and Japanese Patent Laid-Open No. 2010-045343).

SUMMARY

As described above, n-channel FETs are increasing in performance. Inaddition, the development of complementary elements using compoundsemiconductor has been requested to achieve a high element integrationlevel. That is, it is necessary to achieve high carrier mobility and lowgate on-resistance in a p-channel FET as well.

Here, it is necessary to add an impurity such as C or Zn to a p-channelFET, manufactured by selectively etching an epitaxial substrate formedthrough epitaxial growth, to supply holes. In general, however, the morethe impurity, the lower the carrier mobility. Therefore, it has beendifficult to achieve high carrier mobility and low gate on-resistance ina p-channel FET.

The present technology has been devised in light of the foregoing, andit is desirable to provide a semiconductor device and manufacturingmethod of the same that can achieve high carrier mobility and low gateon-resistance in a p-channel FET, manufactured by selectively etching anepitaxial substrate formed through epitaxial growth, so as to achieve ahigh element integration level.

A semiconductor device according to an embodiment of the presenttechnology includes a buffer layer and channel layer. The buffer layeris formed with a semiconductor adapted to produce piezoelectricpolarization. The channel layer is stacked on the buffer layer. Atwo-dimensional hole gas, generated in the channel layer bypiezoelectric polarization of the buffer layer, is used as a carrier ofthe channel layer.

It should be noted that the semiconductor device according to anotherembodiment of the present technology includes a variety of modes such asone implemented in a manner integrated in other device and anotherimplemented in other manner. Further, the present technology can also beachieved as a variety of systems having the semiconductor device, amanufacturing method of the above device, a program adapted to allow acomputer to implement the manufacturing method of the above device, acomputer-readable recording media recording the program and so on.

The present technology produces high-density carriers at theheterointerface between undoped layers, thus providing improved carrier(hole) mobility. This makes it possible to achieve high carrierconcentration, high carrier saturation speed and relatively highbreakdown voltage in a semiconductor device using holes as carriers,thus contributing to low on-resistance, high-speed operation and highwithstand voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of cross-sectionalconfiguration of a semiconductor device according to a first embodiment;

FIG. 2 is a diagram schematically illustrating the crystal structure ofa buffer layer;

FIG. 3 is a diagram describing the band structure of the semiconductordevice;

FIGS. 4A to 4E are diagrams describing the manufacturing method of agate portion relating to FIG. 1;

FIG. 5 is a diagram describing the gate portion formed by impuritydiffusion;

FIGS. 6A to 6F are diagrams describing the manufacturing method of thegate portion relating to FIG. 5;

FIG. 7 is a diagram describing the gate portion formed by vapordeposition of a schottky metal;

FIGS. 8A to 8E are diagrams describing the manufacturing method of thegate portion relating to FIG. 7;

FIG. 9 is a diagram describing the gate portion formed by vapordeposition of the schottky metal via an oxide film;

FIGS. 10A to 10F are diagrams describing the manufacturing method of thegate portion relating to FIG. 9;

FIG. 11 is a diagram illustrating an example of cross-sectionalconfiguration of the semiconductor device according to a secondembodiment; and

FIGS. 12A to 12J are diagrams describing the manufacturing method of thesemiconductor device according to the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present technology will be described below in the following order.

-   (1) Configuration of the First Embodiment of the Semiconductor    Device-   (2) Configuration of the Second Embodiment of the Semiconductor    Device-   (3) Manufacturing Method of the Semiconductor Device According to    the Second Embodiment-   (4) Conclusion

(1) Configuration of the First Embodiment of the Semiconductor Device

FIG. 1 is a diagram illustrating an example of cross-sectionalconfiguration of a semiconductor device 100 according to a firstembodiment. The semiconductor device 100 shown in FIG. 1 includes acompound semiconductor-based p-channel field effect transistor(hereinafter abbreviated as a pFET).

The pFET serving as the semiconductor device 100 is formed byselectively etching an epitaxial crystal growth layer, formed byepitaxial growth, on a substrate 101 serving as a compound semiconductorsubstrate manufactured with compound semiconductor GaAs single crystal.The epitaxial crystal growth layer is formed by stacking, from the sideof the substrate 101, a buffer layer 102, channel layer 103 and gatelayer 104 in this order. A description will be given below of each ofthe layers.

The buffer layer 102 is formed on the substrate 101 and made of asemiconductor that lattice-matches the substrate 101 at a heterojunctioninterface K1 during epitaxial growth. Here, the term “lattice match”refers to growth without any misfit dislocations on the junctionsurface, and may be pseudo-lattice match if a semiconductor layer isformed with a critical film thickness or less before generation ofmisfit dislocations. As described above, it is possible to form thebuffer layer 102 on the substrate 101 by epitaxial growth by providing alattice match between the substrate 101 and buffer layer 102.

It should be noted that at least one semiconductor layer may be stackedbetween the buffer layer 102 and substrate 101 which lattice-matchesboth the semiconductors of the buffer layer 102 and substrate 101 andhas a band gap different from those of the semiconductors of the bufferlayer 102 and substrate 101. Stacking a semiconductor layer between thebuffer layer 102 and substrate 101 as described above provides a largerband gap for improved withstand voltage.

For example, it is possible to stack, for example, GaAlInP quaternaryalloy between the buffer layer 102 and substrate 101. GaAlInP has a bandgap of 1.9 to 2.3 eV, thus providing improved pFET withstand voltage.

Further, the film thickness of the buffer layer 102 is 10 to 1000 nm,and preferably 250 to 1000 nm. As described above, controlling the filmthickness of the buffer layer 102 provides improved FET withstandvoltage. It should be noted that the thicker the buffer layer 102, thehigher the pFET withstand voltage.

Still further, the buffer layer 102 is formed with a semiconductor thatproduces piezoelectric polarization when formed on a GaAs substrate. Theterm “piezoelectric polarization” refers to spontaneous polarizationthat is macroscopically produced by piezoelectric effect and chargeimbalance between positive and negative ions. Piezoelectric effect isattributable to local distortions resulting from the crystal structure.An internal electric field is generated macroscopically in a givendirection in the buffer layer 102 due to this piezoelectricpolarization.

The internal electric field has at least a vector component in thedirection leading from the substrate 101 to the channel layer 103 in thesemiconductor device 100 according to the present embodiment. Morespecifically, this vector component is predominant in the internalelectric field vector of the buffer layer 102. As a result, the bufferlayer 102 is positively charged at the heterojunction interface K1 withthe substrate 101 and negatively charged at a heterojunction interfaceK2 with the channel layer 103. InGaP is among semiconductors thatproduce such piezoelectric polarization. It should be noted that InGaPused as the buffer layer 102 may contain an addition of an impurity orno addition at all.

FIG. 2 is a diagram describing piezoelectric polarization in InGaPepitaxially grown on a GaAs substrate. As illustrated in FIG. 2, whenInGaP is epitaxially grown on the (001) crystal plane of the GaAssubstrate, a natural superlattice structure is formed in the <111>direction. This InGaP natural superlattice structure has an orderingvector in the <111> direction of the zinc blende structure.

Therefore, the InGaP crystal structure changes from a cubic system to atrigonal system, thus leading to piezoelectric effect and spontaneouspolarization. Piezoelectric effect is attributable to local distortionsresulting from the difference in bond length between Ga—P and In—P.Spontaneous polarization is attributable to charge imbalance betweenpositive and negative ions. At this time, a macroscopic internalelectric field is induced in the <111> direction in the epitaxialcrystal growth layer as shown in FIG. 3. That is, an internal electricfield Ei running from the substrate 101 to the channel layer 103 isgenerated in the buffer layer 102.

It should be noted that when InGaP is used as the buffer layer 102, andwhen the indium (In) composition ratio is represented by the formulaIn_(x)Ga_(1-x)P, x=0.51. By adopting this composition ratio, it ispossible to produce significant piezoelectric polarization while at thesame time achieving a lattice match between InGaP of the buffer layer102 and GaAS of the substrate 101 and between InGaP of the buffer layer102 and GaAs of the channel layer which will be described later.

The channel layer 103 is formed on the buffer layer 102. The same layer103 is a semiconductor layer through which a main pFET current flows.The channel layer 103 is formed with a semiconductor whichlattice-matches the buffer layer 102 during epitaxial growth. Byproviding a lattice match between the buffer layer 102 and channel layer103 as described above, it is possible to form the channel layer 103 onthe buffer layer 102 through epitaxial growth.

Further, the channel layer 103 is formed with a semiconductor thatoffers a higher energy level of the valence band than the buffer layer102. Therefore, a potential barrier is formed at the heterojunctioninterface K2. This barrier restricts the migration of holes from thechannel layer 103 to the buffer layer 102.

Still further, the energy level of the valence band at theheterojunction interface K2 is higher than that near the same interfaceK2 on the side of the buffer layer 102 and that at the same interface K2on the side of channel layer 103. Therefore, the energy level of thevalence band at the heterojunction interface K2 changes discontinuouslyor steeply, forming, in the valence band at the heterojunction interfaceK2, an upwardly protruding triangular potential having hole confinementeffect.

Here, holes generated in the channel layer 103 are attracted to theheterojunction interface K2 by the internal electric field of the bufferlayer 102 described above. However, these holes are confined by thetriangular potential formed near the side of the channel layer 103 atthe heterojunction interface K2, causing these holes to be quantized.This allows a two-dimensional hole gas layer (2DHG layer) to be formednear the side of the channel layer 103 at the heterojunction interfaceK2.

The material of the channel layer 103 that meets these conditions whenInGaP is used as the buffer layer 102 is a semiconductor thatlattice-matches this InGaP. Among examples of such a material are GaAs,InGaAs, AlGaAs, InGaAsP and appropriate combinations thereof. Further,an impurity of 1×10¹⁷ atoms/cm³ or less may be added to the channellayer 103, and the film thickness of the same layer 103 is 30 to 150 nm.More preferably, the film thickness of the same layer 103 is 50 to 100nm. It is possible to guarantee the enhancement mode operation of thepFET by controlling the film thickness of the channel layer 103 to fallwithin the above range.

FIG. 3 is a diagram describing the band structure of the semiconductordevice 100. In the example shown in FIG. 3, the buffer layer 102 isformed with InGaP to which no impurity has been added, and the channellayer 103 with GaAs to which no impurity has been added. At this time,as far as the energy level of the valence band is concerned, an energylevel Ev2 of the channel layer 103 is higher than an energy level Ev1 ofthe buffer layer 102.

Further, an energy level Ev3 of the valence band at the heterojunctioninterface K2 between the buffer layer 102 and channel layer 103 ishigher than the energy level Ev1 of the valence band of the buffer layer102 and the energy level Ev2 of the valence band of the channel layer103. As a result, the energy level of the valence band is discontinuousbetween the energy levels Ev1 and Ev3 on the side of the buffer layer102 at the heterojunction interface K2.

On the side of the channel layer 103 at the heterojunction interface K2,on the other hand, the energy level of the valence band changescontinuously. However, the energy level drops steeply from Ev3 to Ev2near the heterojunction interface K2 (within a given distance Ad). As aresult, an upwardly protruding triangular potential P adapted to trapholes is formed near the heterojunction interface K2 on the side of thechannel layer 103. The higher the energy level of the triangularpotential P, the narrower the potential.

Here, the holes generated in the channel layer 103 tend to be attractedto the heterojunction interface K2 by the internal electric field Ei ofthe buffer layer 102. The holes attracted in this way are trapped by thetriangular potential P. Then, the holes trapped by the narrow triangularpotential P are quantized, forming a 2DHG layer on the side of thechannel layer 103 of the heterojunction interface K2.

Here, the density of the two-dimensional hole gas of the 2DHG layer was1×10¹⁷ to 1×10¹⁸ atoms/cm³ when InGaP was used as the buffer layer 102and GaAs as the channel layer, and when the film thickness of the bufferlayer was 10 to 1000 nm, and the film thickness of the channel layer 30to 150 nm. This is equal to or better than that of a HEMT in relatedart. That is, it is clear that a 2DHG equal to or better than that of aHEMT in related art has been produced without performing modulationdoping with an impurity as done for manufacturing a common HEMTstructure in related art.

The semiconductor device 100 according to the first embodiment is, forexample, free from impact of impurity dispersion caused by impuritydiffusion, thus providing a significantly high hole mobility. Therefore,the same device 100 according to the first embodiment provides highcarrier concentration, high carrier saturation speed and relatively highbreakdown voltage, thus contributing to low on-resistance, high-speedoperation and high withstand voltage.

It should be noted that the semiconductor of the channel layer 103 maybe doped with C, Zn or Be as an impurity so long as the concentrationthereof is 1×10¹⁷ atoms/cm³ or less. Further, the semiconductor of thebuffer layer 102 may be doped with C, Zn or Be as an impurity so long asthe concentration thereof is 1×10¹² to 4×10¹⁸ atoms/cm³. It is generallyknown that the concentration of an impurity equal to or greater than1×10¹⁷ atoms/cm³ leads to a steep decline in mobility of the holes,i.e., carriers. However, an impurity whose concentration falls withinthe above range provides further enhanced hole concentration withoutdegrading the hole mobility in the channel layer 103.

Further, according to the experiment conducted by the inventor of thepresent application, the thicker the buffer layer 102, the moretwo-dimensional holes tend to be generated in the channel layer 103.Therefore, increasing the thickness of the buffer layer 102 providesmore carriers generated in the channel layer 103. On the other hand,reducing the thickness of the buffer layer 102 leads to less carriersgenerated in the channel layer 103. That is, it is possible to adjustthe amount of two-dimensional hole gas produced in the channel layer 103by adjusting the thickness of the buffer layer 102.

Further, the buffer layer 102 may be formed with a plurality ofsemiconductor layers that are stacked one on top of another with othertype of semiconductor layer sandwiched between the INGaP layers. Asemiconductor layer formed with a material that has a higher valenceelectron energy level than InGaP and lattice-matches the InGaP layer isused as a semiconductor layer other than the InGaP layers. Amongexamples of such a material are GaAs, InGaAs, AlGaAs, InGaAsP andcombinations of these materials. It should be noted that if the bufferlayer 102 is formed with a plurality of semiconductor layers, an InGaPlayer serving as a piezoelectrically polarized semiconductor is used atleast as a layer joined to the channel layer 103. This allows atwo-dimensional hole gas to be produced by piezoelectric polarization ofthe buffer layer 102.

On the other hand, if the buffer layer 102 is formed with a plurality ofsemiconductor layers, a semiconductor layer formed with a material thathas a higher valence electron energy level than InGaP andlattice-matches the InGaP layer is used as a semiconductor layer otherthan the InGaP layers. Forming the buffer layer 102 with a plurality ofsemiconductor layers provides the same layer 102 with a certain degreeof conductivity. For example, the buffer layer 102 formed with anInGaP/GaAs/InGaP layered film offers better conductivity than thatformed with an InGaP single layer film.

It should be noted that, according to the experiment conducted by theinventor of the present application, even if the buffer layer 102 had amultilayer structure, the number of carriers generated in the channellayer 103 was proportional to the thickness of the buffer layer 102 as awhole. Therefore, even if the buffer layer 102 has a multilayerstructure including semiconductor layers other than the InGaP layers, itis possible to adjust the number of carriers to be produced in thechannel layer 103 by adjusting the thickness of the buffer layer 102 asa whole.

A gate portion 104 a making up a pFET gate is formed on the channellayer 103, and a drain electrode 105 and source electrode 106 are formedwith the gate portion 104 a therebetween. Here, the gate portion 104 acan be formed by a variety of methods such as a combination of epitaxialgrowth and selective etching, impurity diffusion, vapor deposition of aschottky metal and vapor deposition of a schottky metal via an oxidefilm.

Here, letting the leak current of the gate portion manufactured by acombination of epitaxial growth and selective etching be denoted by I1,the leak current of the gate portion manufactured by impurity diffusionby I2, the leak current of the gate portion manufactured by vapordeposition of a schottky metal by I3, and the leak current of the gateportion manufactured by vapor deposition of a schottky metal via anoxide film by I4, the relationship I4<I1=I2<I3 holds. The leak currentshould preferably be small. Therefore, the gate portion 104 a should beideally manufactured by vapor deposition of a schottky metal via anoxide film. It should be noted, however, that selective etching ispractically preferred for a compound semiconductor because of thedifficulties involved in forming and controlling an oxide film in acompound semiconductor.

In the example shown in FIG. 1, the gate layer 104 serving as an n-typesemiconductor layer for a gate area is formed on the channel layer 103by epitaxial growth, and the gate portion 104 a is formed by selectivelyetching the gate layer 104.

The gate layer 104 shown in FIG. 1 can be, for example, a GaAs, InGaP orAlGaAs layer or a combination thereof to which an n-type impurity suchas Si has been added at a concentration of 1×10¹⁷ to 1×10¹⁹ atoms/cm³.If an n-InGaP layer is used as the gate layer 104, x=0.49 inIn_(x)Ga_(1-x)P. This provides a lattice match between the channel layer103 and gate layer 104. If an n-AlGaAs layer is used as the gate layer104, x=0.1 to 0.5 in Al_(x)Ga_(1-x)As. This provides reduced leakcurrent in the gate portion 104 a. It should be noted that, morepreferably, if an n-AlGaAs layer is used as the gate layer 104, x=0.25in Al_(x)Ga_(1-x)As. This provides reduced leak current while at thesame time keeping the ratio of Al, a material that becomes readilyoxidized, to a minimum.

The film thickness of the gate layer 104 is not specifically limited ifthe same layer 104 is formed by stacking an InGaP layer and GaAs layerin this order from the side of the channel layer 103. Due toprocess-related problems, however, it is practical to set the filmthickness of the n-InGaP layer used as a stop layer to 10 to 50 nm, andthat of the n-GaAs layer to 50 to 200 nm.

FIGS. 4A to 4E are diagrams describing the manufacturing method of thegate portion 104 a of the semiconductor device 100 shown in FIG. 1. InFIGS. 4, the gate portion 104 a is formed by coating the gate layer withresist (FIG. 4A), followed by making an opening by exposing anddeveloping the resist in the area other than that where the gate portion104 a is to be formed (FIG. 4B), etching the gate layer 104 other thanthe area where the gate portion 104 a is to be formed so as to leaveonly the gate portion 104 a unremoved (FIG. 4C), and peeling off theresist (FIG. 4D).

Then, the drain electrode 105 and source electrode 106 arevapor-deposited with the gate portion 104 a therebetween in such amanner as to come into ohmic contact with the channel layer 103, thusmanufacturing the semiconductor device 100 (FIG. 4E).

FIG. 5 illustrates the semiconductor device 100 having the gate portion104 a formed by impurity dispersion. In the semiconductor device 100shown in FIG. 5, the gate portion 104 a is formed with an n-typeimpurity diffused in the channel layer 103. At this time, the distancebetween the channel layer 103 and 2DHG layer has been set to 50 to 100nm by adjusting the impurity diffusion depth. This makes it possible toadjust the pFET threshold voltage, i.e., the current characteristic withrespect to the gate voltage. For example, the smaller the distancebetween the gate portion 104 a and 2DHG layer, the easier it is toperform enhancement mode operation, and the larger the distancetherebetween, the easier it is to perform depletion mode operation.Further, an n-type impurity used to form the gate portion 104 a is, forexample, Si, S, Se, Te, Sn or Ge, and the impurity concentration (donorconcentration Nd) thereof is 1×10¹⁷ to 1×10¹⁹ atoms/cm³.

FIGS. 6A to 6F are diagrams describing the manufacturing method of thegate portion 104 a of the semiconductor device 100 shown in FIG. 5. InFIG. 6, the gate portion 104 a is formed by depositing a SiN film on thechannel layer 103 by CVD (chemical vapor deposition) for passivation andcoating the SiN film with resist (FIG. 6A), followed by making anopening by exposing and developing the resist in the area for the gateportion 104 a (FIG. 6B), making an opening by etching the SiN film inthe area for the gate portion 104 a with the resist as a mask (FIG. 6C),diffusing the impurity into the channel layer 103 from the opening ofthe SiN film (FIG. 6D), and peeling off the resist and removing the SiNfilm (FIG. 6E).

Then, the drain electrode 105 and source electrode 106 arevapor-deposited with the gate portion 104 a therebetween in such amanner as to come into ohmic contact with the channel layer 103, thusmanufacturing the semiconductor device 100 (FIG. 6F).

FIG. 7 illustrates the semiconductor device 100 having the gate portion104 a formed by vapor deposition of a schottky metal. In thesemiconductor device 100 shown in FIG. 7, the gate portion 104 a isformed by directly schottky-joining a gate electrode to the channellayer 103. The schottky metal used as the gate portion 104 a is, forexample, Al, Zr, Hf, Gd, Fe, Nd, Sn, Yb, Au, Ti or Ni.

FIGS. 8A to 8E are diagrams describing the manufacturing method of thegate portion 104 a of the semiconductor device 100 shown in FIG. 7. InFIG. 8, the gate portion 104 a is formed by coating the channel layer103 with resist (FIG. 8A), followed by making an opening by exposing anddeveloping the resist in the area where the gate portion 104 a is to beformed (FIG. 8B), vapor-depositing a schottky metal thereon (FIG. 8C),and lifting off the schottky metal vapor-deposited in the area otherthan the gate area by peeling off the resist (FIG. 8D).

Then, the drain electrode 105 and source electrode 106 arevapor-deposited with the gate portion 104 a therebetween in such amanner as to come into ohmic contact with the channel layer 103, thusmanufacturing the semiconductor device 100 (FIG. 8E).

FIG. 9 illustrates the semiconductor device 100 having the gate portion104 a formed by vapor deposition of a schottky metal via an oxide film.In the semiconductor device 100 shown in FIG. 9, the gate portion 104 ais formed by vapor-depositing a schottky metal on an insulating filmthat is deposited on the channel layer 103. An oxide film such as Al₂O₃,HfO, Ga₂O or GaON is formed to the thickness of 10 to 30 nm for use asthe insulating film. On the other hand, the schottky metal used as thegate portion 104 a is, for example, Al, Zr, Hf, Gd, Fe, Nd, Sn, Yb, Au,Ti or Ni.

FIGS. 10A to 10F are diagrams describing the manufacturing method of thegate portion 104 a of the semiconductor device 100 shown in FIG. 9. InFIGS. 10A to 10F, the gate portion 104 a is formed by depositing aninsulating film on the channel layer 103, followed by coating theinsulating film with resist (FIG. 10A), making an opening by exposingand developing only the resist in the gate area (FIG. 10B),vapor-depositing a schottky metal thereon (FIG. 10C), and lifting offthe schottky metal vapor-deposited in the area other than the gate areaby peeling off the resist (FIG. 10D).

Then, the areas, on both sides of the gate portion 104 a, are etcheduntil the channel layer 103 is reached (FIG. 10E), followed byvapor-depositing the drain electrode 105 and source electrode 106respectively in the openings formed by etching in such a manner as tocome into ohmic contact with the channel layer 103, thus manufacturingthe semiconductor device 100 (FIG. 10F).

The gate portion 104 a of the semiconductor device 100 according to thepresent embodiment can be manufactured by a variety of methods asdescribed above, thus allowing formation of the same portion 104 a bythe method best suited for the intended purpose.

(2) Configuration of the Second Embodiment of the Semiconductor Device

A description will be given next of another embodiment using the abovepFET. Among suitable embodiments using the above pFET are complementaryinverter and level shift logic. In a second embodiment described below,a description will be given by taking, as an example, a case in whichthe above pFET is used as a complementary inverter.

FIG. 11 is a diagram illustrating an example of cross-sectionalconfiguration of a semiconductor device 200 according to the secondembodiment. The semiconductor device 200 shown in FIG. 11 is acomplementary inverter having a compound semiconductor-based pFET andn-channel field effect transistor (hereinafter referred to as an nFET)formed on the same substrate. The pFET used for this complementaryinverter corresponds to the pFET according to the first embodiment. ThepFET according to the second embodiment which will be descried belowpermits substitution or combination of the features of the pFETaccording to the first embodiment as appropriate.

In the semiconductor device 200 according to the second embodiment,layers 202 to 205, i.e., epitaxial layers adapted to form an n-channelfield effect transistor (nFET), and layers 206 to 211, i.e., epitaxiallayers adapted to form a p-channel field effect transistor (pFET), areformed in this order on a compound semiconductor substrate 201, i.e., aGaAs single crystal substrate, by epitaxial growth.

The semiconductor device 200 has two areas, a first area A1 in which thepFET is formed, and a second area A2 in which the nFET is formed. Thefirst and second areas A1 and A2 are formed on the same single compoundsemiconductor substrate by processing (e.g., etching and doping) anepitaxial substrate in a proper sequence. The epitaxial substrate islayered and formed on the compound semiconductor substrate 201 byepitaxial growth.

Both the first and second areas A1 and A2 have an epitaxial crystalgrowth layer for forming the nFET. This epitaxial crystal growth layerincludes the first buffer layer 202, first barrier layer 203, firstchannel layer 204 and second barrier layer 205 in this order from theside of the compound semiconductor substrate 201 as illustrated in FIG.11. It should be noted that either the first barrier layer 203 or secondbarrier layer 205 may be omitted as necessary.

The first buffer layer 202 is a semiconductor layer inserted between thecompound semiconductor substrate 201 and first barrier layer 203 tobuffer the difference in lattice constant between the two layers. Thesame layer 202 is, for example, an AlGaAs layer to which a p-typeimpurity has been added. It should be noted that the first buffer layer202 may be an undoped GaAs layer, and a variety of materials can be usedas the same layer 202 so long as they can buffer the difference inlattice constant between the compound semiconductor substrate 201 andfirst barrier layer 203.

The first barrier layer 203 is formed, for example, by stacking a firstcarrier supply layer 203 a and first high resistance layer 203 b in thisorder from the side of the compound semiconductor substrate 201.

The first carrier supply layer 203 a is a semiconductor layer adapted tosupply electrons, i.e., carriers, to the first channel layer 204. Thesame layer 203 a is, for example, an AlGaAs layer of approximately 3 nmin thickness to which a high concentration of Si, i.e., an n-typeimpurity, of 1.0×10¹² to 4.0×10¹⁸ atoms/cm³ has been added.

The high resistance layer 203 b is a semiconductor layer formed toprovide an excellent heterojunction interface between the first carriersupply layer 203 a and first channel layer 204. The same layer 203 b is,for example, an AlGaAs layer of approximately 3 nm in thickness to whichno impurity has been added.

The first channel layer 204 is a semiconductor layer through which amain nFET current flows. The same layer 204 is, for example, an InGaAslayer of 5 to 15 nm in thickness to which no impurity has been added.

The second barrier layer 205 is formed, for example, by stacking asecond high resistance layer 205 a and second carrier supply layer 205 bin this order from the side of the compound semiconductor substrate 201.

The second high resistance layer 205 a is a semiconductor layer formedto provide an excellent heterojunction interface between the firstchannel layer 204 and the second carrier supply layer 205 b that isformed on the second high resistance layer 205 a. The same layer 205 ais, for example, an AlGaAs layer of approximately 3 nm in thickness towhich no impurity has been added.

The second carrier supply layer 205 b is a semiconductor layer adaptedto supply electrons, i.e., carriers, to the first channel layer 204. Thesame layer 205 b is, for example, an AlGaAs layer of approximately 6 nmin thickness to which a high concentration of Si, i.e., an n-typeimpurity, of 1.0×10¹² to 4.0×10¹⁸ atoms/cm³ has been added.

The schottky layer 206 is a semiconductor layer adapted to form anexcellent heterojunction interface between the same layer 206 and asecond buffer layer 207 formed on the schottky layer 206. The same layer206 is, for example, an AlGaAs layer of 70 to 200 nm in thickness towhich a low concentration of Si, i.e., an n-type impurity, of 1.0×10¹⁰to 5.0×10¹⁷ atoms/cm³ has been added.

In the second area A2, the schottky layer 206 has a p-type gate area 220in which Zn, i.e., a p-type impurity, has been diffused. An insulatingfilm 260 made of a silicon nitride film is formed on the top surface ofthe schottky layer 206 in the second area A2. An opening 226 is formedin the insulating film 260 to connect a device external to thesemiconductor device 200 and the schottky layer 206, with a gateelectrode 223 formed in the opening 226.

The gate electrode 223 includes a metal electrode formed by stacking,for example, titanium (Ti), platinum (Pt) and gold (Au) in this order.An ohmic contact is established between the gate electrode 223 and thep-type gate area 220 formed thereunder. A source electrode 221 and drainelectrode 222 are formed with the gate electrode 223 therebetween. Thesource electrode 221 and drain electrode 222 penetrate the insulatingfilm 260, thus establishing an ohmic contact with the schottky layer206.

Next, as for the first area A1 in which the pFET is formed, a secondbuffer layer 207, second channel layer 208, gate leak prevention layer209, n-type first gate layer 210 and n-type second gate layer 211 areprovided in the order of the hierarchical structure of layers used forthe second area A2.

The second buffer layer 207 is a semiconductor layer inserted betweenthe schottky layer 206 and second channel layer 208 to buffer thedifference in lattice constant between the two layers. The same layer207 is, for example, an InGaP layer of 10 to 1000 nm in thickness towhich no impurity has been added. It should be noted that an impuritymay be added to the second buffer layer 207.

The second channel layer 208 is a semiconductor layer through which amain pFET current flows. The same layer 208 is formed on the secondbuffer layer 207 and is, for example, a GaAs, InGaAs, AlGaAs or InGaAsPlayer or a combination thereof of 30 to 150 nm in thickness to which noimpurity has been added. Of course, in addition to these materialsabove, a variety of materials can be used as the second channel layer208 as with the channel layer 103 in the first embodiment describedabove so long as they lattice-match the second buffer layer 207 and havea higher energy level of the valence band than the second buffer layer207.

The gate leak prevention layer 209 is a semiconductor layer formedbetween the second channel layer 208 and n-type gate layer and adaptedto prevent gate leak current. The same layer 209 is, for example, anAlGaAs layer of 0 to 50 nm in thickness to which no impurity has beenadded. It should be noted that the gate leak prevention layer 209 may beomitted as necessary.

An n-type gate area 250 is formed on the gate leak prevention layer 209.The same area 250 is narrower than the layers 207 to 209 that are formedon the hierarchical structure of the layers 202 to 206 in the first areaA1. The same area 250 has a two-layer structure made up of the n-typefirst gate layer 210 and n-type second gate layer 211 stacked in thisorder from the side of the compound semiconductor substrate 201.

The n-type first gate layer 210 is, for example, an InGaP layer of 10 to50 nm in thickness to which an n-type impurity such as Si has been addedat a concentration of 1×10¹⁷ to 5×10¹⁹ atoms/cm³.

The n-type second gate layer 211 is, for example, an GaAs layer of 50 to200 nm in thickness to which an n-type impurity such as Si has beenadded at a concentration of 1×10¹⁷ to 5×10¹⁹ atoms/cm³.

An insulating film 260 made of a silicon nitride film is formed on theside surfaces of the second buffer layer 207, second channel layer 208,gate leak prevention layer 209 and n-type gate layers and on the topsurfaces of the gate leak prevention layer 209 and n-type gate layers.

In the insulating film 260 formed on the top surface of the gate leakprevention layer 209, openings 230 are formed with the n-type gatelayers therebetween and at a distance from these layers that are stackedon the gate leak prevention layer 209. Source and drain electrodes 231made of a metal are formed in the openings 230.

Each of the source and drain electrodes 231 includes a metal electrodeformed by stacking, for example, titanium (Ti), platinum (Pt) and gold(Au) in this order. An ohmic contact is established between the sourceand drain electrodes 231 and source and drain areas 232 formedthereunder, respectively.

The source and drain areas 232 are diffusion areas formed by diffusingZn, i.e., an impurity, into the gate leak prevention layer 209 from theopenings 230 and transforming part of the gate leak prevention layer 209and second channel layer 208 into p-type areas. That is, the source anddrain areas 232 are formed in such a manner as to penetrate the gateleak prevention layer 209 and extend to part of the second channel layer208.

It should be noted that an element isolation area 240 is formed in theboundary area between the first and second areas A1 and A2 to penetratethe layers 201 to 206. The element isolation area 240 is formed, forexample, by implanting B (boron) ions.

As described above, a pFET having a pn junction gate is formed in thefirst area A1, and an nFET having a pn junction gate in the second areaA2. This allows formation of complementary FETs on the same substrate.Both of these complementary FETs, and the pFET, in particular, can beoperated in enhancement mode and offer reduced leak current, thuscontributing to high-speed operation.

(3) Manufacturing Method of the Semiconductor Device According to theSecond Embodiment

A description will be given next of the manufacturing method of thesemiconductor device 100 according to the second embodiment withreference to FIGS. 12A to 12J. FIG. 12A is a vertical cross-sectionalview illustrating the layered structure of the semiconductor device 100formed by epitaxially growing each of the layers made primarily of GaAsmaterials on a GaAs single crystal substrate, for example, by metalorganic chemical vapor deposition (MOCVD).

In order to form the layered structure shown in FIG. 12A, a GaAs layerto which no impurity has been added is epitaxially grown on the compoundsemiconductor substrate 201 made of GaAs single crystal, thus formingthe first buffer layer 202 of approximately 200 nm in thickness.

Next, an AlGaAs layer to which a high concentration of Si, i.e., ann-type impurity, of 1.0×10¹² to 4.0×10¹² atoms/cm³, and, for example,3.0×10¹² atoms/cm³, has been added is epitaxially grown on the firstbuffer layer 202, thus forming the first carrier supply layer 203 a ofapproximately 3 nm in thickness.

Next, an AlGaAs layer to which no impurity has been added is epitaxiallygrown on the first carrier supply layer 203 a, thus forming the firsthigh resistance layer 203 b of approximately 3 nm in thickness. Thefirst carrier supply layer 203 a and first high resistance layer 203 bmake up the first barrier layer 203. The aluminum (Al) composition ratioof the first barrier layer 203 represented by the formulaAl_(1-x)Ga_(x)As, is, for example, Al_(0.2)Ga_(0.8)As by setting x=0.1to 0.5.

Next, an InGaAs layer to which no impurity has been added is epitaxiallygrown on the first high resistance layer 203, thus forming the firstchannel layer 204 of 5 to 15 nm in thickness. By setting x=0.51, theindium (In) composition ratio of the first channel layer 204 representedby the formula In_(1-x)Ga_(x)As provides a narrower band gap than forthe first barrier layer described above.

Next, an AlGaAs layer to which no impurity has been added is epitaxiallygrown on the first channel layer 204, thus forming the second highresistance layer 205 a of approximately 2 nm in thickness.

Next, an AlGaAs layer to which a high concentration of Si, i.e., ann-type impurity, of 1.0×10¹² to 4.0×10¹² atoms/cm³ has been added isepitaxially grown on the second high resistance layer 205 a, thusforming the second carrier supply layer 205 b of approximately 6 nm inthickness.

The second high resistance layer 205 a and second carrier supply layer205 b make up the second barrier layer 205. The aluminum (Al)composition ratio of the second barrier layer 205 represented by theformula Al_(1-x)Ga_(x)As is, for example, Al_(0.2)Ga_(0.8)As by settingx=0.1 to 0.5. This provides the second barrier layer with a wider bandgap than that of the first channel layer 204.

Next, an AlGaAs layer to which a low concentration of Si, i.e., ann-type impurity, has been added, is epitaxially grown on the secondcarrier supply layer 205 b, thus forming the schottky layer 206 of 70 to200 nm in thickness.

Next, an InGaP layer to which no impurity has been added is epitaxiallygrown, thus forming the second buffer layer 207 of 10 to 1000 nm inthickness.

Next, a GaAs layer to which no impurity has been added is epitaxiallygrown on the second buffer layer 207, thus forming the second channellayer 208 of 30 to 150 nm in thickness.

Next, an AlGaAs layer to which no impurity has been added is epitaxiallygrown on the second channel layer 208, thus forming the gate leakprevention layer 209 of 0 to 50 nm in thickness. A “0” thickness isgiven because the gate leak prevention layer 209 is not typicallynecessary. The aluminum (Al) composition ratio of the gate leakprevention layer 209 represented by the formula Al_(1-x)Ga_(x)As is, forexample, Al0.2Ga0.8As by setting x=0.1 to 0.5.

Next, an InGaP layer to which an n-type impurity such as Si has beenadded at a concentration of 1×10¹⁷ to 5×10¹⁹ atoms/cm³ is epitaxiallygrown on the gate leak prevention layer 209 or second channel layer 208,thus forming the n-type first gate layer 210 of 10 to 50 nm inthickness.

Next, a GaAs layer to which an n-type impurity such as Si has been addedat a concentration of 1×10¹⁷ to 5×10¹⁹ atoms/cm³ is epitaxially grown onthe n-type first gate layer 210, thus forming the n-type second gatelayer 211 of 50 to 200 nm in thickness. The n-type first gate layer 210and n-type second gate layer 211 make up the n-type gate layer. Itshould be noted that the epitaxial growth described above is conductedat a temperature of approximately 600° C.

Next, as illustrated in FIG. 12B, the n-type second gate layer 211 andn-type first gate layer 210 are selectively removed in this order, forexample, by photolithography technique and wet or dry etching technique.This etching forms the n-type gate area 250 in the first area A1.

Next, as illustrated in FIG. 12C, the gate leak prevention layer 209 andsecond channel layer 208 are selectively removed in this order, forexample, by photolithography technique and wet or dry etching technique.At this time, the InGaP second buffer layer 207 serves as an etchingstop layer, thus minimizing the overetching of the second area A2. Thisprevents the etching from affecting the on-resistance andoff-capacitance of the nFET.

Then, as illustrated in FIG. 12D, the second buffer layer 207 isselectively etched using, for example, hydrochloric acid. As a result ofthese etching steps, the n-type gate area 250 is stacked on or above thelayers 207 to 209 that are left unremoved in the first area A1, with thelayers 207 to 211 all removed by etching in the second area A2.

Next, as illustrated in FIG. 12E, the insulating film 260 made of asilicon nitride film is formed to a thickness of 100 to 500 nm on theexposed surface of the substrate by plasma CVD.

Next, as illustrated in FIG. 12F, the openings 230 are formed in theinsulating film 260 to form the source and drain areas in the first areaA1, and an opening 224 is also formed in the insulating film 260 to formthe gate area in the second area A2. The openings 230 and 226 are formedby photolithography technique and anisotropic etching based, forexample, RIE (Reactive Ion Etching) technique.

Next, as illustrated in FIG. 12G, Zn, i.e., an impurity, is diffused allthe way through the gate leak prevention layer 209 and halfway throughthe second channel layer 208 in the thickness direction via the openings230 of the insulating film 260. Zn is also diffused halfway through theschottky layer 206 in the thickness direction via the opening 226. Zn isintroduced and diffused through the openings 230 and 226 by heating thesubstrate at a temperature of approximately 600° C. in a gaseousatmosphere containing diethyl zinc (Zn(C₂H₅)₂) and arsine (AsH₃). Thisallows the p-type source and drain areas 232 to be formed in the firstarea A1 and the p-type gate area 220 to be formed in the second area A2.

It should be noted that the depth of Zn diffusion through the opening226 in the second area A2 should preferably be approximately 10 nm ormore away from the top surface of the first channel layer 204.Alternatively, Zn may be injected by ion implantation.

Next, as illustrated in FIG. 12H, the element isolation area 240 isformed to electrically isolate the first and second areas A1 and A2 fromeach other. The same area 240 is formed from the schottky layer 206 to adepth reaching the bottom of the first carrier supply layer 203 a. Theelement isolation area 240 can be formed, for example, by implanting Bions.

Next, as illustrated in FIG. 12I, a metal film is deposited on thesubstrate surface, followed by selective removal thereof byphotolithography and etching techniques, thus forming the source anddrain electrodes 231 in the first area A1 and the gate electrode 223 inthe second area A2 at the same time.

The metal film is formed by depositing titanium (Ti), platinum (Pt) andgold (Au) respectively to thicknesses of 30 nm, 50 nm and 120 nm, forexample, by electron beam vapor deposition. This allows an ohmic contactto be established between the p-type source and drain areas 232 and thep-type gate area 220 into which Zn has been diffused.

Further, as illustrated in FIG. 12J, a protective film 265 made of aninsulating material is deposited on the substrate surface, followed byformation of the opening 224 and an opening 225 in the protective film265 and insulating film 260 in such a manner as to sandwich the gateelectrode 223 in the second area A2.

Then, gold-germanium (AuGe) alloy and nickel (Ni) are deposited tothicknesses of approximately 160 nm and 40 nm respectively on thesubstrate surface by resistance heating, followed by selective removalthereof by photolithography and etching techniques, thus forming thesource electrode 221 and drain electrode 222. An ohmic contact isestablished between the same electrodes 221 and 222 and the n-typeschottky layer 206.

It should be noted that when the openings 224 and 225 are formed in theprotective film 265 and insulating film 260, an opening may be formed atthe top of the n-type gate area 250 in the first area A1 at the sametime, thus allowing the source and drain electrodes 221 and 222 and thegate electrode to be formed in the second area A2 at the same time.

The manufacturing method described above permits manufacture of acomplementary inverter by allowing formation of the pFET and nFET whosestructures are shown in FIG. 11 on the same substrate at the same time.

(4) Conclusion

The semiconductor device described above includes the buffer layer 102and channel layer 103. The buffer layer 102 is formed with asemiconductor adapted to produce piezoelectric polarization. The channellayer 103 is stacked on the buffer layer 102. A two-dimensional holegas, generated in the channel layer 103 by piezoelectric polarization ofthe buffer layer 102, is used as a carrier of the channel layer 103.This provides high carrier mobility and low gate on-resistance in ap-channel FET manufactured by selectively etching an epitaxialsubstrate, thus contributing to a high element integration level.

It should be noted that the present technology is not limited to theabove embodiments and modification example but includes configurationsresulting from mutual substitution or altered combination of theconfigurations disclosed in the above embodiments and modificationexample, those resulting from mutual substitution or altered combinationof well-known technologies and the configurations disclosed in the aboveembodiments and modification example and so on. Further, the technicalscope of the present technology is not limited to the above embodimentsbut is applied to the features set forth in the scope of the appendedclaims, and equivalents thereof.

Still further, the present technology may have the followingconfigurations.

(1) A semiconductor device including:

a buffer layer formed with a semiconductor adapted to producepiezoelectric polarization; and

a channel layer stacked on the buffer layer, in which

a two-dimensional hole gas, generated in the channel layer bypiezoelectric polarization of the buffer layer, is used as a carrier ofthe channel layer.

(2) The semiconductor device of feature (1), in which

the semiconductor adapted to produce piezoelectric polarization in thebuffer layer is InGaP.

(3) The semiconductor device of feature (1) or (2), in which

the channel layer is formed with a semiconductor having a higher energylevel of the valence band than the buffer layer.

(4) The semiconductor device of any one of features (1) to (3), in which

a two-dimensional hole gas is generated in the channel layer by thepiezoelectric polarization in an amount proportional to the thickness ofthe buffer layer.

(5) The semiconductor device of any one of features (1) to (4), in which

the buffer layer is formed with a plurality of semiconductor layers thatlattice-match each other, in which

of the plurality of semiconductor layers, the layer provided adjacent tothe channel layer is formed with a semiconductor adapted to producepiezoelectric polarization.

(6) The semiconductor device of any one of features (1) to (5), in which

the channel layer is formed by stacking a semiconductor thatlattice-matches the semiconductor adapted to produce piezoelectricpolarization at least once.

(7) The semiconductor device of any one of features (1) to (6), in which

the semiconductor of the channel layer is doped with an impurity at aconcentration of 1×10¹⁷ atoms/cm³ or less, and in which

the semiconductor of the buffer layer is doped with an impurity.

(8) The semiconductor device of any one of features (1) to (7), in which

the buffer layer is stacked on a compound semiconductor substrate, inwhich

at least one semiconductor layer is stacked between the buffer layer andcompound semiconductor substrate, and the semiconductor layerlattice-matches both the semiconductors of the buffer layer and compoundsemiconductor substrate and has a band gap different from those of thesemiconductors of the buffer layer and compound semiconductor substrate.

(9) The semiconductor device of any one of features (1) to (8)including:

a gate formed with an n-type semiconductor stacked on the channel layer.

(10) The semiconductor device of any one of features (1) to (8)including:

a gate formed by diffusing an n-type impurity into the channel layer.

(11) The semiconductor device of any one of features (1) to (8)including:

a gate formed with a schottky metal joined to the channel layer.

(12) The semiconductor device of any one of features (1) to (8)including:

a gate formed by joining a schottky metal to a gate oxide film stackedon the channel layer.

(13) A complementary semiconductor device on which the semiconductordevice of any one of features (1) to (12) and an n-type field effecttransistor are formed on the same compound semiconductor substrate.

(14) A level shift circuit manufactured by using the semiconductordevice of any one of features (1) to (12).

(15) A semiconductor device manufacturing method including:

forming a compound semiconductor base portion;

forming, on the base portion, a buffer layer by stacking a semiconductorthat lattice-matches the compound semiconductor of the base portion andproduces piezoelectric polarization;

forming, on the buffer layer, a channel layer by stacking asemiconductor that lattice-matches the semiconductor of the buffer layerand produces a two-dimensional hole gas by piezoelectric polarization;

forming a gate on the channel layer; and

forming a drain and source with the gate therebetween on the channellayer.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2012-112033 filed in theJapan Patent Office on May 16, 2012, the entire content of which ishereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a buffer layerformed with a semiconductor adapted to produce piezoelectricpolarization; and a channel layer stacked on the buffer layer, wherein atwo-dimensional hole gas, generated in the channel layer bypiezoelectric polarization of the buffer layer, is used as a carrier ofthe channel layer.
 2. The semiconductor device of claim 1, wherein thesemiconductor adapted to produce piezoelectric polarization in thebuffer layer is InGaP.
 3. The semiconductor device of claim 1, whereinthe channel layer is formed with a semiconductor having a higher energylevel of the valence band than the buffer layer.
 4. The semiconductordevice of claim 1, wherein a two-dimensional hole gas is generated inthe channel layer by the piezoelectric polarization in an amountproportional to the thickness of the buffer layer.
 5. The semiconductordevice of claim 1, wherein the buffer layer is formed with a pluralityof semiconductor layers that lattice-match each other, and of theplurality of semiconductor layers, the layer provided adjacent to thechannel layer is formed with a semiconductor adapted to producepiezoelectric polarization.
 6. The semiconductor device of claim 1,wherein the channel layer is formed by stacking a semiconductor thatlattice-matches the semiconductor adapted to produce piezoelectricpolarization at least once.
 7. The semiconductor device of claim 1,wherein the semiconductor of the channel layer is doped with C, Zn or Beas an impurity at a concentration of 1×10¹⁷ atoms/cm³ or less, and thesemiconductor of the buffer layer is doped with C, Zn or Be as animpurity at a concentration of 1×10¹² to 4×10¹⁸ atoms/cm³.
 8. Thesemiconductor device of claim 1, wherein the buffer layer is stacked ona compound semiconductor substrate, and at least one semiconductor layeris stacked between the buffer layer and compound semiconductorsubstrate, and the semiconductor layer lattice-matches both thesemiconductors of the buffer layer and compound semiconductor substrateand has a band gap different from those of the semiconductors of thebuffer layer and compound semiconductor substrate.
 9. The semiconductordevice of claim 1 comprising: a gate formed with an n-type semiconductorstacked on the channel layer.
 10. The semiconductor device of claim 1comprising: a gate formed by diffusing an n-type impurity into thechannel layer.
 11. The semiconductor device of claim 1 comprising: agate formed with a schottky metal joined to the channel layer.
 12. Thesemiconductor device of claim 1 comprising: a gate formed by joining aschottky metal to a gate oxide film stacked on the channel layer.
 13. Acomplementary semiconductor device on which a semiconductor device andan n-type field effect transistor are formed on the same compoundsemiconductor substrate, the semiconductor device including a bufferlayer formed with a semiconductor adapted to produce piezoelectricpolarization, and a channel layer stacked on the buffer layer, wherein atwo-dimensional hole gas, generated in the channel layer bypiezoelectric polarization of the buffer layer, is used as a carrier ofthe channel layer.
 14. A level shift circuit manufactured by using asemiconductor device including a buffer layer formed with asemiconductor adapted to produce piezoelectric polarization, and achannel layer stacked on the buffer layer, wherein a two-dimensionalhole gas, generated in the channel layer by piezoelectric polarizationof the buffer layer, is used as a carrier of the channel layer.
 15. Asemiconductor device manufacturing method comprising: forming a compoundsemiconductor base portion; forming, on the base portion, a buffer layerby stacking a semiconductor that lattice-matches the compoundsemiconductor of the base portion and produces piezoelectricpolarization; forming, on the buffer layer, a channel layer by stackinga semiconductor that lattice-matches the semiconductor of the bufferlayer and produces a two-dimensional hole gas by piezoelectricpolarization; forming a gate on the channel layer; and forming a drainand source with the gate between the drain and source on the channellayer.